A number of recent advances have led to increased utilization of hydraulic mount assemblies to support internal combustion engines in motor vehicles. The assemblies normally include two mounting members connected together by an elastomeric body. A damping chamber filled with liquid is formed in this body. An orifice plate divides this chamber into two cavities. The first cavity is formed between the orifice plate and the elastomeric body. The second cavity is formed between the orifice plate and a separate expandable diaphragm received within a base member of one of the mounting members.
Engine and/or road vibrations at prescribed frequencies and amplitudes that are transmitted to the mount assembly cause the elastomeric body to alternatively compress and stretch. This causes a change of volume in the first cavity. As the first cavity decreases in volume on compression of the assembly, the damping liquid that fills the cavity is forced through the restricted orifice into the second cavity thereby causing the diaphragm to expand. Upon reversal of the vibration input force, the assembly stretches longitudinally and the first cavity increases in volume. As a result, damping liquid flows back through the restricted orifice into the first cavity, thus causing the diaphragm to contract. This two-step process completes the damping cycle.
Where variable damping is desired (such as high damping at large amplitudes, and no damping at small amplitudes) a decoupler is provided in the orifice plate between the cavities (see, for example, copending commonly assigned U.S. patent application Ser. No. 785,243, filed Oct. 7, 1985 entitled Hydraulic-Elastomeric Mount Displacement Decoupler and now U.S. Pat. No. 4,664,363). The decoupler effects a volume displacement in the cavities so that no fluid flows through the restricted orifice at the prescribed low amplitude levels.
By varying the size and shape of the decoupler and/or the orifice in the decoupler, the engine mount assembly can be tuned so that the hydraulic damping effects a better match to the particular vibrations anticipated. This passive tuning of the mount assembly substantially increases its usefulness and overall effectiveness. To provide additional efficiency in controlling vibrations under an even greater range of conditions, a more recent development provides for active, infinitely variable control (see copending commonly assigned U.S. patent application Ser. No. 929,328 filed Nov. 10, 1986, entitled Variable Hydraulic Elastomeric Mount Assembly). For the engine, which is the main vibration inducing component of the vehicle, the selection of one of these prior art mount assemblies, that is, with or without active control, is the optimum choice.
On the other hand, there are other auxiliary systems or components of the vehicle that also call for vibration isolation if the smoothest and quietest ride is to be obtained. One approach is to provide a completely redesigned hydraulic mount that relies on the exchange of liquid through variable size orifices between substantially concentric chambers (see commonly assigned U.S. patent application Ser. No. 875,187, filed June 17, 1986, entitled Hydraulic Mount). While this prior approach is generally successful for auxiliary equipment, there is technical evidence available that utilizing a design with dual cavities separated by an orifice plate, similar to the basic design used by the engine mounts, would be particularly advantageous.
However, research has proven that the engine mount assemblies are not suitable for these smaller components, such as the muffler and tail pipe, for several reasons. First, the amplitude and frequency range to be damped varies sufficiently to substantially preclude simple adaptation of these prior devices. Secondly, in the case of an exhaust system, a hanger rather than a base type support is better suited to the overall engineering needs to the vehicle. Also, in addition to involving a different dynamic range, there is substantially less weight to be supported. Thus, the mount for auxiliary systems or components calls for a different, but similar, basic design, and advantageously one that is simpler and more economical to manufacture.